The standard benchtop centrifuge is one of the most common instruments in the life science laboratory used ubiquitously for sample preparation in cell biology research and medical diagnostics. Typical sample preparation procedures require multiple centrifugation steps for cell labeling and washing, which can be a time consuming, laborious, and costly process for diagnostics and research. In fact, while assays themselves have widely been miniaturized and automated, sample preparation required for these assays has been identified as a key target for future automation.
Centrifuges perform three critical sample preparation steps that make them so widely used: (i) separation of cells by size/density, (ii) concentration of cells, and (iii) solution exchange. Because centrifuges can perform such disparate functions, realizing these functions in a miniaturized platform has been challenging. Miniaturized microfluidic approaches often successfully implement one or two of these functions. For example, cell separation by size and density has been accomplished by using physical obstacles, external forces, or fluidic forces to guide particles to defined locations in a microchannel for collection at different outlets. While these methods may offer high resolution cell separation, the typical collected liquid volume is similar to the injected liquid volume—that is, no significant concentration is achieved. This large output volume can hinder downstream cell detection platforms that may require scanning large fields of view to observe the cells of interest or leads to dilution of biomolecules of interest if collected cells must be lysed. Thus, a method of concentration must be used in-line with the separation system to reduce the liquid volume for rapid detection and analysis.
There are a variety of techniques for concentrating particles and cells in localized regions with microfluidic systems. Of these, mechanical traps are the most commonly used method that anchors particles and cells to a physical structure and enables multistep perfusion of reagents to perform cell assays on-chip via solution exchange. Often, however, it may be important to release particles and cells on-demand for further downstream analysis. Although successful at concentration and release, cells immobilized in these trap-and-release systems can squeeze through traps and become damaged when operated at higher volumetric throughput, thereby limiting concentration factors to below what is necessary for concentration of rare cells or dilute cell solutions. Thus, a general purpose miniaturized tool that recapitulates all of the functions and flexibility of a traditional centrifuge has yet to be achieved.
The formation of vortices within a microfluidic structure has been used for focusing and filtration enhancement. For example, Park et al. (Jae-Sung et al., Continuous focusing of microparticles using inertial lift force and vorticity via multi-orifice microfluidic channels, Lab Chip, 9, 939-948 (2009)) discloses a microfluidic device used in experiments that focuses rigid microparticles using a series of suddenly expanding and contracting channels. At certain flow rates, vortices are formed within the expanded channels. The vortices formed within the expanded channels induce lateral particles migration like a tubular pinch effect. By having a series of these expanded channels along a length of a microchannel, rigid microparticles are able to gradually migrate (i.e., are focused) to opposing sides of the microchannel. Importantly, however, the expanded channels do not trap the particles. Instead, Park et al. discloses a structure that continuously focuses microparticles passing through the device. In Park et al., small diameter (7 μm diameter) polystyrene microspheres were run through a multi-orifice microchannel and trapping of these particles was not observed. Park et al. further observed that larger-sized particles tended to move away from the expanding channel regions where vortices were formed. Park et al. also discloses that particles in the sample should be like rigid spheres for maximal value of the inertial lift force which obviously runs counter to its use with living cells that, by their nature, are generally deformable. Structurally, Park et al. discloses rather small-sized expanding channels that expand outward a distance of around 80 μm with respect to the upstream contracting channel. Further, the length of the expanding channels is also small, disclosed as being 200 μm.
U.S. Patent Application No. 2008/0318324 (Chiu et al.) discloses a biochip for the high-throughput screening of cancer cells. The device uses effusive filtration to segregate tumor cells from a sample of bodily fluid. Effusive filtration refers to filtration configurations where the fluid is dispersed or redistributed by the filtration media or any morphological features inside the flow channel. In Chiu et al., the filtration media are side wall apertures having a width smaller than that of the cell. In one embodiment, Chiu et al. discloses a 1-D channel having an expansion and constriction point to either slow down or speed up flow. Chiu et al. discloses that at high velocities the fluid may become separated to form internal microvortices which aid in the filtration operation by altering fluid flow dynamics. The microvortices, however, do not trap cells passing through the device. Rather, the apertures that line sections of the channel retain larger-sized cells by preventing the same from passing there through. While structures are disclosed that generate vortices for focusing or filtration aiding purposes, these structures are not used to selectively trap cells therein.